dodge lightning

Hey everyone, I just bought a john deere 4310 compact series tractor its a 2003 model. The engine is a 3 cylinder yanmar, natural aspirated. It runs great but I want to turn it up a tad just for fun. There are a couple of screws that I have spotted. One is on the left side on its own little pedistal. There are 3 more in a line directly under the 1 screw. The three screws are spaced evenly w/ the injector lines so i assume this is the indivisual fuel flow to each line. I thought that the screw all by itself would be the one that I wanted to back out but it does absolutly nothing in or out. They are all extremely hard to get to. I havent been able to ajust the 3 screws in a line yet, I thought I would ask you guys first.